Magnetic sheet steel and process for making the same



July 27, 1937. w. E. RUDER 2,033,440

MAGNETIC SHEET STEEL AND PROCESS FOR MAKING THE SAME Filed Aug. 24, 1936 i l l I j I I SILICON CONTENT PERCENT PERCENT SILICON His Attorney.

Patented July 27, 1937 PATENT OFFICE MAGNETIC SHEET STEEL AND PROCESS FOR MAKING THE SAME William E. Ruder, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 24, 1936, Serial No. 97,678

10 Claims.

This application is a continuation in part of my prior application Serial No. 528,460, filed April 7, 1931, and entitled Magnetic sheet material.

The invention relates to magnetic material and more particularly to rolled steel sheets having a relatively high silicon content and a process for making the same.

In the construction of cores for electrical apparatus such as transformers, dynamo electric machines and the like, it is desirable to employ magnetic sheet. steel having low total watt losses. Silicon steel, on account of its satisfactory magnetic properties, is employed extensively for this purpose. Such steel usually contains from about 3.55 to about 4 or 4.25% silicon, the remainder of the composition, except for incidental impurities, being iron. When used as cores in magnetic machinery the steel is usually in the form of sheets or laminations about 14 mils thick.

Heretofore attempts have been made to produce thin rolled steel sheets having a silicon content greater than 5%. These attempts however have been unsuccessful due to the brittleness of steel containing such a high percentage of silicon and to the process employed to produce such sheets. I have discovered a process whereby silicon steel sheets containing up to about 7% silicon may be rolled into thin sheets suitable for use as the laminations in coresof electrical apparatus. I have also discovered that rolled silicon steel containing about 6 to 6 silicon has wholly unexpected and very advantageous magnetic and physical qualities particularly when employed as transformer core laminations.

In the prior processes for manufacturing thin steel sheets of normal silicon content the molten metal is first poured into molds. The cast metal or ingot is allowed to cool substantially to room temperature and is then placed in a furnace, designated a soaking pit, where it is heated for about 4 to 6 hours at atemperature of about 1200 C. to 1250 C. The ingot is then rolled on a bar mill to provide relatively wide bars which are cut to a desired length. These bars are reheated in a pair furnace at a. temperature of about 1000 C. to 1100 C. for about 3 to 4 hours preparatory to rolling the material into sheets of the desired thickness.

In rolling the bars into sheets, it is customary to employ the well known pack rolling process finishing eight sheets to the pack. In this process the bars are first passed singly through the sheet mill. Two sheets thus formed are superimposed and passed through the rolls three or four times. Two additional sheets are then superimposed on the first two sheets and the combined sheets again passed through the rolls 5 a few times. These sheets are then reheated and doubled and given about three passes through the rolls. They are then reheated and given the final pass, reducing the sheets to the desired thickness. Since the temperature of the sheets is reduced considerably during the rolling operation they are usually reheated to about 025 C. after each intermediate rolling treatment. The finishing pass is usually made at a temperature of about 800 C. and with a draft on the rolls of about 15%.

When silicon steel contains more than silicon and is treated in accordance with the prior process it is impossible to roll such material into bars owing to its brittle character. I have found however that if the prior process is modified so that the silicon steel ingot is not permitted at any time to cool substantially to room temperature but is transferred to the soaking pit while still at an elevated temperature and preferably as soon as-the metal in the mold has cooled sufficiently to become solid, it is possible to roll the ingot into bars and the bars into sheets.

For the most satisfactory results the finishing pass should be made with a draft on the rolls or reduction in thickness somewhat less than in prior practice, which draft may be as low as about 5% to 8%, with a temperature as low as about 700 C. to 760 C. However, I have found that a draft on the rolls for the finishing pass of about 10% to 12%, at a temperature of about 850 C. to 900 C., gives a satisfactory percentage of good commercial sheets, and particularly if the high silicon sheets be pack rolled with interleaved softer metal sheets. Shearing the pack and separating the sheets immediately after the final pass are also conducive to increasing the percentage of good commercial sheets.

Silicon steel sheets containing about 6 to 6 /2% silicon have a remarkably low total watt loss when tested at 60 cycles and 10,000 gauss. In addition to low total watt losses, transformer noise due to magnetostriction is greatly reduced when the steel comprising the transformer core consists of silicon steel which contains more than 5% but not more than 7% silicon and is substantially eliminated when the silicon t t of the steel is about 6 to 6 /2% silicon.

The novel features which are characteristic sheets to within a few of my invention are set forth with particularity in the appended claims. The invention itself however will best be understood from reference to the following specification when considered in connection with the accompanying drawing in which Fig. 1 is a curve showing the relation of silicon content of the sheet steel and total watt losses per pound while Fig. 2 is a curve showing the variation in noise, due to magnetostriction, with variation in the percentage of the silicon content of the steel.

In carrying out the present invention, silicon steel containing more than 5% silicon, but not more than 7%, is cast into molds in the usual manner. The cast metal is not allowed to cool substantially to room temperature but instead is transferred to the soaking pit while at a tcmperature several hundred degrees above room temperature, preferably before it has cooled to a temperature below 600 C. For best results the transfer should be made as soon as the ingot has cooled sufficiently to become solid. The ingot is then treated in the usual manner until the finishing passes in the sheet mill. Thus it is held in the soaking pit for about 4 to 6 hours until heated throughout its entire mass to the normal soaking temperature which is a temperature above 1000 C. and preferably between about 1200" C. to 1300" C. The ingot is then, as in the production of low silicon steel sheets, rolled on a bar mill and the bars cut to length and transferred to pair furnaces where they are reheated preparatory to rolling into sheets. The bars are heated for 3 or 4 hours in the pair furnace to a temperature of 1000 to 1100 C. after which they are ready to be rolled in the sheet mill to the desired thickness. However, instead of rolling the ingot on a bar mill it may, if desired, be broken down into billets and rolled into long strips on a, series of roughing and finishing mills. The strip thus formed is cut into plates of desired width and rolled by the usual pack method.

In rolling the metal into sheets, I use the pack rolling process heretofore described, finishing 8 sheets to the pack. The temperature of the sheets at the beginning of each rolling operation. or after the doubling operation, may be as low as 925 C. until the finishing pass. It is important that the sheet furnace temperature be maintained at least at 925 C. although this tem perature may be higher if desired, for example 985 C. If the furnace temperature is allowed to drop materially under 925 C. too much strain is introduced into the rolled sheets.

Instead of rolling down to the desired thickness in what normally would be the finishing passes in the prior process, I may reduce the percent, less than 10%, of the desired thickness and make the finishing pass at a temperature of about 700 to 760 C. with the rolls set for a draft or reduction in thickness of about 5 to 8%. If desired, however, the finishing pass may be made at higher temperatures such as 850 to 900 C. and with the rolls set for a draft or reduction in thickness of about 10 to 12%. particularly if the high silicon steel sheets are pack rolled with interleaved softer metal sheets such as low silicon steel sheets. If the temperature employed on the finishing pass is materially lower than 850 C. the draft on the rolls preferably should be less than 10%. Shearing the pack and separating thesheets immediately after the final pass and preferably before the temperature of the sheets is reduced to about 200 F. are conducive to increasing the percentage of good commercial sheets.

The resistivity of a 6.50 silicon steel sheet is about 85 microhms per cm., whereas 4% silicon steel has a resistivity of about 60 microhms per cm.. The eddy current loss of the higher silicon content steel is therefore unusually low when compared with that of low silicon steel sheets. Steel sheets about 14 mils thick having a silicon content of about 6.50% when tested at 60 cycles and 10,000 gauss show, as indicated in Fig. l of the drawing, a very low total watt loss, the average total watt loss per pound being about 0.35.

While it would be expected that an increase in the silicon content ofrolled silicon steel sheets would be accompanied by a lower eddy current loss owing to increase in resistance of the sheets, it was entirely unexpected that an increase in the silicon content above say 5.5% silicon would cause an abrupt drop in the total watt losses. As indicated on Fig. 1 of the drawing such losses are a minimum in rolled silicon steel sheets containing about 6 to ti silicon. As the silicon content of the steel is increased above ti the total watt losses begin to increase and a 7% rolled silicon steel sheet has a total watt loss not substantially lower than that of steel sheets containing only 5% silicon. Above 7% the loss continues to increase and an 8% rolled silicon steel sheet has a total watt loss far greater than that of 5% silicon steel.

In addition to providing very low total watt losses, rolled silicon steel sheets containing more than 5% but not more than 7% silicon, and preferably about 6 to 636% silicon, have a further advantage when employed as the core material of power transformers. This advantage consists in the substantial elimination of noise in such transformers. Ordinarily, power transformers when in use produce a noise or hum due to cyclic variations in the dimensions of the transformer core in the magnetic field. This variation in dimensions is commonly known as magnetostriction. However, when silicon steel containing more than 5% but not more than 7% is employed as the core material of a power transformer this noise-or hum is greatly reduced and with a core consisting of about 6 to 6 /2% silicon the noise is substantially eliminated, the magnetostriction being positive (expansion) or negative (contraction) as the silicon content is reduced or increased respectively from about 6%%. By employing a transformer core consisting of thin laminations containing about 6 to 6 silicon, it is possible to produce a transformer which is not only substantially noiseless but which is capable of operating with a very high degree of efliciency due to its unusually low total watt loss.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The method of producing-magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting the molten metal in molds to form an ingot, transferring the ingot substantially as soon as it has solidified to a furnace, heating the ingot at a temperature of about 1200 C. to about 1300 C. and thereafter rolling the ingot into relatively thin sheets.

2. The method of producing magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting, the molten metal in molds to form an ingot and transferring the cast ingot when solid but at a temperature above 600 C. to a furnace, heating the ingot at a temperature of about 1200 C. to about 1300 C. and thereafter rolling the ingot into relatively thin sheets.

3'. The method of producing magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting the molten metal in molds to form an ingot, transferring the cast ingot while still heated at a temperature above 600 C. to a furnace, heating the ingot in said furnace at a temperature of about 1200 C. to about 1300 C., rolling the ingot into bars or plates, reheating said bars or plates and rolling them into sheets having a thickness a few per cent greater than the finished thickness by means of a plurality of passes through a sheet mill the temperature of the sheets at the beginning of each series of passes being not appreciably lower than 925 C., heating the sheets to a temperature between 700 C. and 760 C. and thereafter rolling the sheets so as to produce a final reduction in thickness of about 5 to 8%.

4. The method of producing magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting the molten metal in molds to form an ingot, transferring the cast ingot when solid, but at a temperature above 600 C., to a furnace, heating the ingot in said furnace at a temperature of about 1200 C. to about 1300 C., rolling the ingot into sheets having a thickness a few per cent greater than the finished thickness, heating the sheets to a temperature between 700 and 760 C. and thereafter rolling the sheets so as to produce a final reduction in thickness of about 5 to 8%.

5. The method of producing magnetic rolled silicon steel sheets containing more than 5% silicon but not more than 7% silicon which comprises casting the molten metal in a mold to form an ingot and transferring the ingot substantially as soon as it has solidified, to a furnace heated to a temperature of about 1200 C. to about 1300 C., maintaining the ingot in said furnace for about 4to 6 hours, rolling the ingot into sheets having a thickness a few per cent greater than the desired finished thickness of the sheet, heating the sheets to a temperature between 700 C., and 760 C. and thereafter roll-. ing the sheets so as to produce a reduction of about 5 to 8% in the thickness of the sheets.

6. The method of producing magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting the molten metal in a mold to form an ingot, transferring the ingot when solid, but at a temperature several hundred degrees above room temperature, to a furnace, heating the ingot at a temperature of about 1200 C. to about 1300 C., and thereafter rolling the ingot into relatively thin sheets.

7. The method of producing magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting the molten metal in molds to form an ingot, transferring the ingot while it is still at a temperature several hundred degrees above room temperature to a furnace, heating the ingot in said furnace at a temperature of about 1200 C. to 1300 C., rolling the ingot into sheets having a thickness of a few per cent greater than the finished thickness, heating the sheets to a temperature of about 700 C. to 900 C. and thereafter rolling the sheets so as to produce a final reduction in thickness of about 5 to 12%.

8. The method of producing magnetic rolled silicon steel sheets containing more than 5% but not more than 7% silicon which comprises casting the metal in molds to form an ingot, transferring the ingot while it is still several hundred degrees above room temperature to a. furnace, heating the ingot in said furnace to a temperature in the neighborhood of 1200 C. to 1300 C., rolling the ingot into sheets having a thickness a few per cent greater than the desired finished thickness, heating the sheets to a temperature of about 850 to 900 C. and rolling the sheets to produce a final reduction in thickness of about 10 to 12%.

9. A magnetic rolled silicon steel sheet containing more than but not more than 7% of silicon and characterized by a physical structure and magnetic properties such as are obtained when an alloy of this composition is cast in a mold, transferred from the mold to a furnace while at a temperature above 600 C. and heated in said furnace at about 1200 C., and thereafter rolled into thin sheets.

10. A transformer having a core, said core comprising silicon steel laminations, said .lami nations containing about 6 to about ti silicon, said steel laminations being characterized by a. physical structure and magnetic properties such as are obtained when an alloy of this composition is cast in a mold, transferred from the mold to a furnace while at a temperature above 600 C. and heated in said furnace at about 1200 C., and thereafter rolled into thin sheets.

WILLIAM E. RUDER. 

